PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 60. 



The percentage composition, in terms of mineral molecules, is given 

 below. This assumes that all the soda (and potash) go into a jadeite 

 molecule, that there may be present a so-called " babingtonite " 

 (FeO. (Fe,Al)203.4Si02) molecule, that the CaO, MgO, and FeO 

 (above that needed for babingtonite) form diopside, and that the 

 extra FcaOg is present in the pyroxene in solid solution, instead of 

 as the Tschermak molecule, the last assumption being in accordance 

 with the results of as yet unpublished studies by Merwin and me on 

 acmite-aegirite and other pyroxene minerals. The result is calcu- 

 lated to 100 per cent. 



Molecular composition of Tuxtlajade. 



Molecule. 



.Jadeite (NaoO.AlA-^SiO,) 



Babingtonite (FeO.Fe,03.4SiOo^ 



"Wollastonite (CaO.SiOz) 



Enstatite (ilgO.SiOa) 



Ferrosilite (FeO.SiOa) 



Ferric oxide (FeoOg) 



jjadeite, 49.34. 

 Dioppide, 50.efV 



The Tuxtla jade is therefore composed of 49.34 per cent of sodic 

 jadeite (with a slight admixture of potasic jadeite and babingtonite 

 molecules), and 50.66 per cent of an almost purely magnesian diop- 

 side, containing only a little ferrous metasilicate. 



The Tuxtla mineral is, therefore, strictly, a diopside- jadeite, with 

 the two molecules present in almost exactly equal amount. For the 

 benefit of archaeologists, who are not concerned with the niceties of 

 mineralogical nomenclature, it may be said that they would be 

 sufficiently correct to call the material jadeite, as it is a variety of 

 this mineral, and by this name it is thus clearly distinguished from 

 the other jade mineral, nephrite — a very important archaeological, 

 as well as mineralogical, distinction. 



In his interpretation of the analyses of jadeite in the Bishop col- 

 lection, Clarke assumed the presence of a molecule (Ca, Mg, Fe) O. 

 Al203.4Si02, which he called pseudojadeite ; while Penfield, although 

 assuming that very small amounts of CaO, MgO, or FeO could re- 

 place NajO in jadeite, calculated the greater part of these RO 

 oxides as forming diopside in the analyses of the Bishop jadeiteg 

 which he discusses. Merwin and I, as has been said, seem forced to 

 assume the presence of small amounts of the babingtonite molecule, 

 FeO. (AljFe) 203.4810,, which is analogous to Clarke's pseudojadeite, 

 in some acmites and aegirites. But study of the molecular ratios fur- 

 nished by analyses of jadeite makes it clear that such a molecule 



